Known in the art is an acoustic wave element which has a piezoelectric substrate, an IDT (interdigital transducer) electrode (excitation electrode) which is provided on a main surface of the piezoelectric substrate, and a protective layer which covers the IDT electrode (for example, Patent Literature 1 or 2). The protective layer is formed by for example SiO2 and contributes to suppression of corrosion of the IDT electrode, compensation for the change of characteristics of the IDT electrode against temperature, and so on.
Patent Literature 1 points out that in an acoustic wave element which has a protective layer (SiO2 film) such as described above, when the IDT electrode is formed by Al or an alloy which contains Al as a major component, a sufficient reflection coefficient cannot be obtained (paragraph 0009). Further, it proposes to form the IDT electrode by a metal which has a larger density than Al or an alloy which contains this metal as a major component so as to obtain a relatively high reflection coefficient (claim 1 etc.)
Note that, while not matter relating to the reflection coefficient, Patent Literature 1 and 2 propose to improve the adhesion between the IDT electrode and the SiO2 film by forming an adhesion layer between them (Patent Literature 1, paragraph 0011 and Patent literature 2, paragraph 0107). In Patent Literature 1 and 2, the adhesion layer is formed thin so as not to exert an influence upon propagation of the SAW. Specifically, the adhesion layer is made 50 to 100 Å (Patent Literature 1, paragraph 0009) or not more than 1% of the wavelength of the SAW (Patent literature 2, paragraph 0108).
As explained above, Patent Literature 2 proposes to form the IDT electrode by a material which has a higher density than Al. However, from the viewpoints of the electrical characteristics, ease of working, cost of material, etc., use of Al or an alloy which contains Al as a major component as the material of the IDT electrode is sometimes desired. In other words, it is sometimes desired to reduce the restrictions on the material for forming the IDT electrode to thereby improve the degree of freedom in selection of the material. Accordingly, as a method different from that in Patent Literature 2, provision of a method of raising the reflection coefficient is desired.